The thing is electrolytes. In plain water, you dont have any salts in it. If you do the 13k walk, or an exercise that is long (badminton, running, cycling, weight lifting, cricket...), you will loose a lot of salts. The problem is that in a cell, there is a balance beetween the quantity of salt inside and outside the cell. It permit the transport of water in the cell and its called osmosis equilibrium. When you loose a lot of salt, this equilibrium is brooken. The cell will loose water, and wont be able to work properly.
In that way, too much water can do the same thing. You can die. So, we need to drink, but not in excess. The best way to ensure that the balance is respected is to drink sport drink or by adding a little amount of salt to the water. As for myself, i never do a run longer than 40 min without a sport drink.
smoky, i can tell you that not just athlete can have this problem. You can die drinking too much water and sitting in your chair watching the TV. And you can die sitting in that same chair because of deshydratation.
Furthermore, deshydratation and lack of salt are related. Plain water is consider as a diuretic. If you add salts in small amount, it will be retained longer in the blood and aborb faster.
Like i said, it is always a question of balance... if blood salts drop (due to sweet), water level drop. Then you drink plain water? blood water level will increase a little but wont be able to stay because the salt level is too low = deshydratation. You decide to flood yourself with plain water to compensate = hyponatremia = death.
So please, Drink on SPORTS DRINK and train hard, especially in summer.
